Voltage controlled oscillators are known to include resonant circuits comprising inductors and capacitors. While ideally these reactive elements are selected to have a high effective quality factor Q, they inherently include also some resistivity values, and therefore are susceptible to noise generation. In addition, the electrical parameters of these components are known to vary with applied voltages, temperature changes, and aging. The foregoing properties of these electrical resonators cause undesirable frequency variations of the generated signal. In applications where it is important to maintain very high spectral purity of the generated signal, it is therefore desirable to utilize oscillators which do not include elements susceptible to noise. Well known examples of such stable frequency oscillators are crystal oscillators in which the frequency of oscillation is controlled by electromechanical properties of piezoelectric crystal resonators, rather than by electrical parameters. As it is well known in the art, crystal resonators have very large values of Q, and vary very little with temperature or age. While crystal oscillators have the desired stability of the frequency of oscillations, that frequency is determined by the mechanical properties of the crystal and thus cannot be dynamically changed within a desired wide frequency range.
One example of a system where it is desirable to generate a signal having high spectral purity, while providing frequency variations within a wide range, is an audio sampling rate signal generator system, generally utilized in digital audio signal processors. When audio signals are recorded in digital form utilizing a magnetic tape recording/playback device, a sampling signal is generated to sample the audio signal, and the resulting samples are recorded. During playback the audio sampling signal is utilized to decode the data. For correct decoding of the information content the sampling signal rate must be stable and must be locked to the other clock signals utilized in the various associated playback devices. However, when the audio signal has been recorded at a certain speed of the recording medium, and it is played back at a different speed, the playback sampling signal frequency must be changed proportionally so that the played back signal can be correctly decoded.
A particular application where the playback speed is different from the recording speed occurs in video signal recording/playback devices utilizing special effects, when the signal is played back at speeds which slightly differ from the original recording speed. For example, it may be desired to reduce the playback time of commercials, or to make a particular video program to appear shorter or longer. To obtain the foregoing, the color television signal is reproduced utilizing time base correctors with automatic scan tracking, which produces slow motion pictures by reproducing the same picture more than once, or fast motion by periodically deleting pictures, in a well known manner. However, while the foregoing techniques are practical for processing video signals, they are not applicable for reproduction of audio signals accompanying the picture information. In case the audio signal is played back at a medium speed which differs from the medium speed during recording, its original frequency is distorted, and an unpleasant audible distortion results. Therefore, when the video signal is played back utilizing special effects, it is customary to pass the accompanying off-tape audio signal through an external device which returns it to its normal frequency and intonation.
Audio signals which are recorded digitally by high quality recording/reproducing systems, such as for broadcast applications, are known to utilize a predetermined audio sampling rate for each channel. During special effects provided for the accompanying video signal that sampling rate must be increased or decreased to conform to the change in playback speed with respect to the original recording speed. The performance of these high quality digital audio systems depends in large part on the spectral purity of the sampling signal, that is on the stability of the sampling signal rate.